Among the many kinds of photographic cameras which are known in the art, there are cameras wherein the diaphragm aperture is preselected, and then the shutter speed (duration of exposure) is automatically set to give an optimum exposure value as a result of prevailing illumination of the subject being photographed. Likewise there are known cameras operating the opposite way, wherein the user first preselects and manually sets the shutter speed (time of exposure) and then the automatic mechanism automatically sets the diaphragm aperture in accordance with the illumination of the subject. In both of these types of cameras it sometimes happens, however, that the preselected factor will not result in an optimum exposure value for any possible setting within the available range of the other factor which is to be set automatically. For example, in a given camera, the available range of setting the diaphragm aperture may be from f/2.8 to f/16. Suppose, for example, that the photographer preselects the shutter speed as 1/200th of a second. The automatic setting mechanism will then attempt to find and set an appropriate diaphragm aperture value which will give an acceptable exposure under prevailing light conditions when the shutter speed is 1/200. But if the subject being photographed is only faintly illuminated, it may well be that a shutter speed of 1/200 is too fast to obtain adequate exposure even if the diaphragm is opened wide to its extreme limit of f/2.8. Under these conditions, since the diaphragm cannot be opened any farther to a larger aperture, it is necessary to use a slower shutter speed, such as 1/100 instead of 1/200. This is automatically accomplished by the mechanism of the present invention. The present invention also operates to shift the shutter speed to a faster speed (shorter duration of exposure) if an acceptable exposure cannot be obtained when the diaphragm aperture is stopped down to its smallest aperture. Likewise, the invention operates similarly with the reverse arrangement, where the diaphragm aperture is selected manually and the shutter speed is then set automatically. If it is not possible for the automatic setting mechanism to select a shutter speed which will give an optimum exposure, within the available range of shutter speeds and at the preselected diaphragm aperture, then the mechanism of the present invention will shift the diaphragm aperture to such extent as may be necessary to produce an optimum exposure.
As well known in the art, there are three exposure factors which must be considered when taking a picture. One is the factor of shutter speed (that is, duration of each exposure), another is the diaphragm aperture or stop, and the third is the factor of film sensitivity or film speed. For purposes of the present discussion, the two basic exposure factors are shutter speed and diaphragm aperture, which frequently need to be varied from one exposure to the next, whereas the factor of film sensitivity is relatively constant, as it is always the same for any one loading of the camera with a roll of film, and needs to be changed only when the camera is loaded with a fresh roll of film having a different sensitivity or film speed.
An object of the present invention is the provision of an improved camera which will operate in the manner above indicated, i.e., in which one exposure factor is normally preselected and set manually, and the other of the two main exposure factors is then selected and set automatically without changing the first factor, but which will actually change the preselected first factor automatically if such a change is necessary in order to achieve an optimum exposure.
Another object is to provide a camera of the above stated kind with minimum possible circuitry expense, so that it can be of cost-favorable and space-saving construction and so that furthermore by a simple switching measure, preferably by a simple changeover switch, the camera can be converted from the operation manner of priority time setting or of priority diaphragm stop setting into the manner of operation of an automatic time device or an automatic diaphragm device, or into the manner of operation for manual diaphragm and time setting.